Efforts have been made to develop optical metamaterials for manipulation of optical waves by propagating them through metamaterials with spatially varying refractive indices, including the cases when the refractive indices are less than 1 or even negative. These metamaterials can be used in applications such as optical cloaking and optical lensing. In the cloaking application, the invisibility sought may be partial at a specific frequency, or over a broader band of frequencies. The cloaking is designed to decrease scattering of electromagnetic wave on an object contained within a space, including “back scattering” by the object and also reducing its shadow (“forward scattering”). If both, back and forward, scatterings are reduced, the combination of the cloak and the object contained therein has a resemblance to free space. When the phrase “cloaking” or the like, is used herein, the effect may be that the object may not be visible or may appear in a distorted or attenuated form, or the background obscured by the object may be distorted or partially obscured. Several known methods exist which attempt to achieve electromagnetic concealment of objects. For example, it is possible to use a series of cameras to project an image to an observer of what he would see if an object in question were not blocking his view path. As a result, the observer does not realize that the object is present. This method, however, relies on the use of active components, and depends heavily on the relative positioning of the object, cameras and observer at all times.
There have also been efforts at preventing the detection of an object using “stealth” technology, where the objective is to make the object as invisible as possible in the reflection or back scattering direction. One means of doing this is to match the impedance of the stealth material to that of the electromagnetic wave at the boundary, but where the material is strongly attenuating to the electromagnetic waves. In this way, the energy backscattered from the object within the stealth material is strongly attenuated on reflection, and there is minimal electromagnetic reflection at the boundary within the design frequency range. This is typically used in evading radar in military applications. Shadowing may not be a consideration in stealth technology. In order to exploit electromagnetism, materials are commonly used to control and direct the electromagnetic fields. For example, a glass lens in a camera directs the rays of light to form an image, metal cages are used to screen sensitive equipment from electromagnetic radiation, and various forms of “black bodies” are utilized to prevent unwanted reflections. Although such structures can provide a reduced or altered electromagnetic signature, because they involve either the scattering of incident waves away from the target or absorbing incident waves, the objects which they hide are still detectable in transmission.
More recently, there have been efforts to conceal objects by the use of materials referred to as “metamaterials”, which are composite materials that display properties beyond those available in naturally occurring materials. Typically, metamaterials are constructed with two or more materials on the microscopic (sub-micron for optical applications) level, that exhibit a refractive index with a value that is smaller than 1, zero, or negative. Metamaterials with spatially varying refractive index might be used to form optical cloaking devices. The inhomogeneous 2D or 3D patterns of refractive index in such material may cause the electromagnetic radiation incident thereon to bend in a controlled manner so that so it flows around a “hidden” object inside, such that the object becomes invisible. There is a special trajectory of light which avoids objects by propagating around them and returning to their original direction with undisturbed wave-fronts, which may be referred to as an optical cloak.
Metamaterials may also be useful in other systems to control electromagnetic radiation. As the electromagnetic radiation propagates by taking a shortest optical path given by product of the geometrical distance and the local refractive index of the medium, materials with complex and/or reconfigurable distribution of refractive indices offer an opportunity to control light by using specific patterns of the refractive indices, for example, gradients of the refractive indices may focus or defocus light beams by bending their trajectories.
Therefore, there is a need to create an optical metamaterials which allow one to create and control the spatial pattern of refractive indices.